Kate Warne (August 28, 1833 – January 28, 1868) was an American law enforcement officer best known as the first female detective in the United States, for the Pinkerton National Detective Agency. She also had a role in uncovering the 1861 Baltimore Plot against President-elect Abraham Lincoln, recruiting female agents for the Pinkerton Agency, and conducting intelligence work for the Union during the American Civil War.

Very little is known about Kate Warne prior to her working for Allan Pinkerton, except that she was born in Erin, Chemung County, New York and was a widow by age 23. An obituary following her death described her parents as "honest and industrious people" and stated that they were poor, resulting in her taking over many of the household duties. Pinkerton, in his book The Spy of the Rebellion (1883), described her as a commanding person, with "clear cut, expressive features" and called her a "slender, brown-haired woman, graceful in her movements and self-possessed." He added that her features, "although not what could be called handsome [beautiful], were decidedly of an intellectual cast" and said that her face was "honest."

In 1856, at the age of 23, Warne walked into the Pinkerton Detective Agency in response to an advertisement in a local newspaper, and requested a job as a detective. Pinkerton was initially hesitant to hire her. However, Warne convinced him that her undercover skills would be helpful. Pinkerton company records reported Pinkerton as declaring "It is not the custom to employ women detectives!" and noted that Warne argued her point of view, adding that "women have an eye for detail and are excellent observers."

Warne's arguments swayed Pinkerton, who employed her as his first female detective. Pinkerton soon had a chance to put Warne to the test. In 1858, Warne was involved in the case of Adams Express Company embezzlements, where she was successfully able to bring herself into the confidence of the wife of the prime suspect, Mr. Maroney. She thereby acquired valuable evidence, leading to the husband's conviction. Mr. Maroney was an expressman living in Montgomery, Alabama. The Maroneys stole $50,000 from the Adams Express Company. With Warne's help, $39,515 was returned. Mr. Maroney was convicted and sentenced to ten years in Montgomery, Alabama. In 1860, Allan Pinkerton put Warne in charge of his new Female Detective Bureau, which was based in Chicago, Illinois. For the Bureau, she recruited women such as Elizabeth H. Baker and Hattie Lawton to be agents.

In 1861, Allan Pinkerton was hired by Samuel H. Felton, president of the Philadelphia, Wilmington and Baltimore Railroad, to investigate secessionist activity and threats of damage to the railroad in Maryland. Pinkerton went to work placing agents at various points in Maryland to investigate this potential activity. As the investigation proceeded, Pinkerton became aware that the activity in Maryland did not just end with the railroad; it also included the president-elect, Abraham Lincoln. Pinkerton received permission to continue his investigation and focus on the possible assassination plot. Warne was one of five agents sent to Baltimore, Maryland, on February 3, 1861, to investigate the hotbed of secessionist activity.

During the investigation, evidence unveiled a plot to assassinate Lincoln on his way to take office. Under the aliases Mrs. Cherry and Mrs. M. Barley (M.B.), Warne tracked suspicious movement among the Baltimore secessionists. It was in part through her undercover work in the guise of a wealthy Southern lady visiting Baltimore "with a thick southern accent", allowing her to infiltrate secessionist social gatherings in the Baltimore area. This included the classy Barnum Hotel, also known as Barnum's City Hotel, allowing her to verify a plot to assassinate Lincoln and develop details of how the assassination was going to occur. Pinkerton had agents across Maryland, but it was Warne specifically who supplied many of the key details that led Pinkerton to believe that the plot was imminent. Warne had befriended secessionists in Maryland and collected many details about the plot to assassinate Lincoln.

The president-elect, Abraham Lincoln, was traveling from his home in Springfield, Illinois, to the capital via a train tour that was to stop at notable cities along the way. His published program showed that Lincoln's last leg of the journey was from Harrisburg, Pennsylvania, to Washington, D.C. Due to the configuration of the rail system, all southbound trains required a transfer to be made in Baltimore. The tracks from points north ended at Calvert Street and the tracks heading south started at Camden Street (now the Camden Yards station). The distance between these two stations was about a mile by carriage ride. The secessionist plot was said to be that when Lincoln was passing through part of Calvert St. Station, a row or fight would occur, resulting in police officers rushing out, leaving Lincoln "entirely unprotected and at the mercy of a mob of Secessionists" that would then surround him. It was further alleged that a small steamship had been chartered, sitting in a nearby river, on which the murderers would flee, and travel immediately to the state of Virginia.

After seeing the pieces of the plot coming together, Pinkerton directed Kate Warne to take the 5:10 evening train to New York City on the morning of February 18. Once there, she was to set up a meeting with Norman B. Judd and give him a letter from Pinkerton outlining the known details of the assassination attempt. After receiving the details of the Baltimore Plot from Warne, Judd set up a meeting between himself, Pinkerton, and Lincoln on February 21. At this meeting, Lincoln was doubtful of the existence of an assassination plot or that if such a plot existed that it should be taken seriously.